An intelligent disk drive records and reproduces data on concentric tracks located on each of a plurality of disk surfaces. A transducer head mounted on an actuator arm is moved over a rapidly spinning disk to a position over one of the tracks and transfers data when a targeted data block location is under the head. Each track has a plurality of data sectors, generally corresponding to data blocks, dispersed around the track. In a preferred configuration, the track also has servo sectors dispersed at regular intervals around the track to provide for sampled signal servo control of the transducer head position. The regular intervals of servo sectors when viewed on the disk surface form "wedges" or divided segments of the track with the data sectors disposed between servo sectors.
Conventionally the data sectors are numbered in sequential order from a reference point called the index. The index for a given disk surface is frequently designated by a special servo sector. The sequentially numbered data sectors are known as "physical sectors."
When viewed by a host computer, the intelligent disk drive is frequently treated as a storage device for a large linear string of arbitrarily sized "blocks" of data. Conventionally a data block is 512 bytes in length and, because the host computer need not be aware of the actual physical layout of the drive, the data blocks are termed "logical data blocks." In transactions over a commonly employed interface such as the SCSI protocol, the host and disk drive transfer data to one another in logical data block units. The means for referencing a given logical data block is its sequential address in the string known as a "logical block address" or LBA.
Within the disk drive logical blocks are recorded in the data sectors. A disk drive control microprocessor determines a plan for placing or "mapping" the logical blocks onto specific tracks and data sectors on the disk surfaces. When the host retrieves a logical data block, the disk drive consults the plan to retrieve the data from a specific physical location i.e. surface/track/sector.
The host computer usually requests data transfers in groups of blocks having sequential LBA's rather than single blocks. When large groups of blocks are transferred, the transfer is generally termed "sequential." When many small groups of data blocks are transferred where the groups are non-sequential, the transfer is generally termed "random." The mapping of logical data blocks on the disk surface can greatly impact the measured performance of the drive, therefore great care is taken to optimize the mapping to account for mechanical and electronic delays in accessing data.
Mechanical delays include the time required to move the actuator arm to a specific track or "cylinder" known as a "seek" delay. The other primary mechanical delay is the (average) time required for a sector to pass under the head once the head is positioned over a track. This is termed a "rotational" delay governed by the disk spin rate.
When the data is located on a different surface than that presently being accessed, a "head switch" must be performed to select a new head associated with the surface and to allow the electronic channel to adapt to the gain requirements of the new head and to allow the head to settle on the track. Thus when considering the time required to access data from a given reference point, the seek, rotational, and head switch delays are key performance determinants.
FIG. 1 is a partial cross sectional view of a prior art disk drive 100 having two disks 200 mounted on spindle 110 and corresponding surfaces 202,204,206, and 208. Data tracks 250 are disposed on the surfaces. Conventionally, a large array of sequential blocks may be written to the data tracks by writing to track 250a until all sectors have been written. A head switch to surface 204 is then performed as indicated by dashed line 310, and data continues to be written to track 250b, directly underlying track 250a. When track 250b is filled, a head switch to surface 206 and track 250c is performed as indicated by dashed line 315. Track 250c is filled, then a head switch to surface 208 and track 250d is performed, indicated by dashed line 320. Tracks 250a, 250b, 250c, and 250d form a "cluster" in that they represent a set of tracks in which consecutive logical data blocks are written. When track 250d is filled, a combination seek to track 250e and head switch to surface 202, indicated by the dash-dot line 325, is performed in order to continue the sequential writing process. In this manner sequential logical data blocks are mapped to the surfaces by writing to each track of the cluster in order before proceeding to the next track. The mapping convention was chosen because the delay required for a head switch in the prior art is significantly less than that required to perform a single track seek, thereby maximizing performance.
In current disk drives, the need to store vastly increasing amounts of data has caused data tracks on the disk surfaces to be significantly more closely spaced than in the prior art, thereby reducing the mechanical delay to seek between adjacent tracks. Newer heads, the electronics channel, and the methods of recording data have increased in complexity which, in combination with the more closely spaced tracks, has effectively reversed the traditional ratio between time required for a head switch and time required to perform a single track seek. The combined effect is to create a need for an improved sequential data block mapping scheme which takes into account the reversed ratio.